1. Field of the Invention
The present invention relates to a tuning type laser oscillator apparatus capable of varying the oscillation wavelength of a laser oscillator, and further relates to laser radar and laser communication systems incorporating such laser oscillator apparatus.
2. Description of the Prior Art
Since the oscillation frequency of a laser beam produced within a laser oscillator is essentially determined by only the wavelength corresponding to the energy difference responsible for the laser transition between an upper level and a lower level, the laser oscillator can oscillate, in principle, at discrete wavelengths only. Practically, however, the upper and lower levels participating in the laser transition have attendant energy bands, with the result that the oscillation usually develops within a spectrum interval extending to some extent. The laser oscillator is allowed to oscillate only when such a frequency in the longitudinal mode as determined by the optical length of a resonator comes into the spectrum interval and exceeds an oscillation threshold. Accordingly, it is possible to vary the oscillation frequency of the laser by varying the frequency in the longitudinal mode within the range of the spectrum interval. A conventional tuning type laser oscillator has been produced on the basis of this principle. In general, the number of oscillation spectra due to electronic transitions is one or several at most and, in addition, wavelengths of the several spectra are spaced far apart from each other in many cases. Thus, in a tuning type laser oscillator based on the electronic transistions a pair of reflecting mirrors constituting a resonator are coated wih a dielectric multi-layer film which can select an intended wavelength with a small loss, and these mirrors are associated with a slight variation with the optical length of the resonator.
On the other hand, another laser oscillator based on vibrational or rotational transitions of molecules, prevents the selection of individual branches with the dielectric multi-layer film because of the narrow spacing of the emitted branches. It has been a general measure for solving this problem that one reflecting mirror of the resonator is replaced by a diffraction grating or, alternatively, a prism is incorporated into the resonator to endow the reasonator itself with a wavelength selecting ability of high resolution and, for varying of the oscillation frequency, the frequency in the longitudinal mode was varied by slightly varying the optical length of the resonator.
In the conventional application of the laser oscillator to an optical heterodyne system two similar laser oscillators each having a diffraction grating or prism have been employed, wherein one is operated for a transmitter and the other for a local oscilator, and the difference between the two oscillators is determined to be equal to the intermediate frequency of the heterodyne system. However, such a heterodyne detector requires the two laser oscillators to be oscillated at the same branch and therefore, it is necessary to use at least two different wavelength selecting operations. Although the relation between the inclination of the diffraction grating relative to the optical axis of the resonator and the wavelength of the diffracted beam can be preselected, it is difficult to give the two wavelength selecting mechanisms the same ability and to suppress an error in the selecting operation caused by repeated usage to a practically permissible extent.